(1) Field of the Invention
The present invention relates to semiconductor processing for integrated circuits, and more particularly relates to a method for accurately calibrating a constant-angle reflection-interference spectrometer (CARIS) used to measure photoresist thickness on wafers. Monitor wafers having patterned photoresist layers of varying photoresist thickness are used to generate swing curves. These swing curves, resulting from standing waves formed from interference during optical exposure, are manifested by sinusoidal variations in the critical dimensions (CD) as a function of photoresist thickness when the photoresist is developed. The minima in these curves are used to select a monitor wafer with a more accurately known photoresist thickness. The monitor wafer is used to determine more accurate Cauchy coefficients for calibrating the CARIS tool which is then used to measure more accurate photoresist thickness for product wafers.
(2) Description of the Prior Art
Semiconductor processing for forming integrated circuits requires a series of processing steps. These processing steps include the deposition and patterning of a variety of material layers, such as insulating layers, polysilicon layers, metal layers, and the like. The material layers are typically patterned using a patterned photoresist layer as an etch mask that is patterned over the material layer. The photoresist layer is deposited to the desired thickness by spin coating. The photoresist is then subjected to monochromatic radiation (light) through a photomask or reticle to a desired dose, and then developed in a photoresist developer to form the photoresist etch mask.
As the minimum feature sizes on the semiconductor circuits decrease to submicrometer dimensions, it becomes necessary to more accurately control the critical dimensions (CD). However, the CD of the photoresist image is dependent on numerous processing parameters, such as the photoresist type, radiation dose for exposing the photoresist, development time, and photoresist thickness. Therefore, to control the photoresist CD, it is necessary to accurately determine the photoresist thickness.
Typically the resist thickness is measured using a constant-angle reflection-interference spectrometer (CARIS). The radiation reflected off the resist layer and off the substrate results in fringes which are a function of the wavelength. However, since the optical dispersion (as measured by the refractive index n) is also a function of radiation wavelength, it is necessary to determine the index as a function of wavelength. In the visible range of the radiation, the dependence of refractive index n on wavelength is described by the empirical Cauchy equation EQU n=n.sub.1 +n.sub.2 /(lamda).sup.2 +n.sub.3 /(lamda).sup.4
where n is the refractive index, n.sub.1, n.sub.2, and n.sub.3 are the Cauchy coefficients, and lamda is the wavelength.
Another problem that can complicate the CD control is the swing effect. This occurs when the photoresist is exposed using monochromatic radiation. The constructive and destructive interference between the incident radiation and reflected radiation from the wafer surface result in standing wave edge profiles in the photoresist image when the resist image is developed. This effect manifests itself as a sinusoidal variation in the resist linewidth image as a function of the thickness of the photoresist. This is best depicted by the curve 1 in FIG. 1 of the prior art where the variation in photoresist image size (linewidth) W in micrometers (um) is plotted as a function of photoresist thickness T in um, and is commonly referred to as the CD swing curve. The exposure dose D, in milliJoules/cm.sup.2, to just clear the resist during development as a function of resist thickness T (um) is depicted by curve 2 of prior art FIG. 2. A plot of this curve also displays the characteristic swing effect. This swing effect shows that the CD of a photoresist linewidth can vary by about 0.1 um for a linewidth having a CD of about 0.5 um, which is a wide variation and is undesirable. One method of minimizing this swing effect by the prior art is to use antireflective coatings (ARCs) to minimize the reflected radiation.
Several techniques for measuring film thicknesses have been reported. One method of measuring the thickness and refractive index of films is described in U.S. Pat. No. 5,646,734 to Venkatesh et al. Another method is described in U.S. Pat. No. 4,670,650 to Matsuzawa et al. in which Auger electron spectroscopy is used for measuring the latent image prior to developing the photoresist and therefore avoids additional manufacturing cost. Mumola in U.S. Pat. No. 5,337,150 describes a method for measuring thin film thicknesses using a correlation reflectometer and a reference wafer having various thicknesses.
However, there is still a need in the semiconductor industry to provide more accurate Cauchy coefficients for the conventional CARIS instrument.